This invention relates to ceramic insulation tiles and, more particularly, to a method a making a fiber-reinforced ceramic insulation tile that provides increased permeability therethrough.
As is well known in the art, ceramic bodies in the form of tiles have been widely used in a number of aeronautical and aerospace applications to insulate structures against high temperatures. For example, such ceramic tiles have been used to protect the Space Shuttle fuselage against the high temperatures associated with reentry into the Earth""s atmosphere. Ceramic tiles are preferred in such applications, as they possess the ability to withstand these high temperatures, have excellent thermal insulation, and can withstand thermal shock. These materials are well known in the art and include, for example, AETB (alumina enhanced thermal barrier), HTP (high thermal performance), FRCI (fibrous refractory composite insulation) and LI (LOCKHEED insulation) materials.
One known type of ceramic tile is a porous ceramic made by pressing together fibers of one or more ceramic materials, such as alumina enhanced thermal barrier (AETB) material which is well known in the art and more fully described in Leiser et al., xe2x80x9cOptions for Improving Rigidized Ceramic Heatshieldsxe2x80x9d, Ceramic Engineering and Science Proceedings, 6, No. 7-8, pp. 757-768 (1985) and Leiser et al., xe2x80x9cEffect of Fiber Size and Composition on Mechanical and Thermal Properties of Low Density Ceramic Composite Insulation Materialsxe2x80x9d, NASA CP 2357, pp. 231-244 (1984). As those skilled in the art will understand, the tile may be formed from other fibrous low-density silica-based materials including, for example, LOCKHEED insulation (LI), high thermal performance (HTP), and fibrous refractory composite insulation (FRCI), which is well known in the art and more fully described in U.S. Pat. No. 4,148,962, the disclosure of which is incorporated herein by reference.
In fabricating tiles like AETB, fibers of an insulating material, such as silica, alumina borosilicate, and alumina, are mixed with water to form a slurry. The slurry is deposited into a casting tower where the water is drained and the silica fibers are subjected to compressive forces to form a raw block of insulation material having a cross-sectional area that may range from 144 square inches to almost 576 square inches depending upon the dimensions of the casting tower. The raw block is then dried in an oven and subsequently fired (sintered) to bond the fibers of the insulating material together. Thereafter, tiles are formed from the fired block through conventional machining processes wherein tiles of a desired shape are cut from the solid block.
This ceramic tile is resistant to damage from thermal shock and thermal cycling. However, it is relatively soft and can be damaged by external impact and wear forces. To lessen such damage, it is known to apply protective coatings to the exterior surface of the ceramic insulation. Examples of these protective coatings are disclosed in U.S. Pat. Nos. 5,702,761 and 5,928,775, issued to DiChiara, Jr. et al. and U.S. Pat. No. 5,079,082, issued to Leiser, et al., the disclosures of which are incorporated herein by reference. However, known ceramic tiles suffer from certain disadvantages in that although they have a generally high porosity, which makes them light weight, their permeability is very low, which generally restricts the flow of fluid therethrough.
In many applications, it would be preferable to introduce a cooling fluid, such as bleed air, through the tile to aid in maintaining a proper temperature of the tile. As described above, conventional ceramic tiles are very porous, yet they are not very permeable. This inhibits the use of bleed air to cool the tile and, thus, limits the use of ceramic tiles to only certain applications. Accordingly, there exists a need in the relevant art to provide a ceramic tile that is sufficiently permeable to enable a cooling fluid to flow therethrough.
In the aeronautical industry, there has been a growing trend to produce space vehicles that fly longer in the atmosphere in an aircraft-like configuration. Additionally, there is a growing trend to insulate aircraft structures from the engines using insulation tile materials. These changes in mission applications change the way insulating materials are used. For instance, space vehicles, such as the space shuttle, use insulation tiles as a pure insulator. These tiles are sized in thickness to accommodate a high temperature heat pulse for a short period of time (seconds), such as that which occurs during a short reentry maneuver. This thickness ensures that the inner mold line or interior surface of the tile does not reach a temperature where the adhesive used to bond the interior surface of the tile to the space vehicle structure is adversely effected.
Typically, the maximum temperature along the inner mold line of the insulating tile occurs at some point (usually minutes) after the high temperature heat pulse has gone from the surface. As space vehicles operate longer in the atmosphere, like an aircraft, the thermal design needs of the insulator change. That is, vehicles that fly longer in the atmosphere will approach a steady state heating condition of the tile rather than a heat temperature heat pulse. Therefore, this steady exposure to intense heat must not expose the inner mold line to excessive heat; as such excessive heat may adversely reduce the useful life of the adhesive. These insulating tiles are highly porous (approximately 90% porosity) but have low permeability (approximately 25 microns in size), which limits the throughflow of fluid.
Accordingly, it is a principal object of the present invention to provide a method of making a fiber reinforced ceramic tile having a high permeability to increase the mass flow of a cooling fluid therethrough.
It is another object of the present invention to provide a method of making a fiber reinforced ceramic tile having a high permeability, yet a porosity and density that is generally equal to a standard ceramic tile.
It is still another object of the present invention to provide a method of making a fiber reinforced permeable ceramic tile that is not susceptible to the shortcomings of the prior art designs.
The above and other objects are provided by a method of making a permeable, fiber-reinforced ceramic tile in accordance with the teachings of the present invention. The method of making a permeable fiber-reinforced ceramic body according to the principles of the present invention includes mixing an organic particulate with silica fiber, alumina fiber, alumina borosilicate fiber, a dispersant, and water to produce a slurry of fibrous ceramic material. The slurry is then placed within a mold and vacuum pressure is applied thereto to substantially remove the water so as to form a fibrous ceramic body. The fibrous ceramic body is then dried and sintered to a temperature sufficient to bond the ceramic material together to form a porous ceramic article. Simultaneously, the fibrous ceramic body is heated to a temperature sufficient to generally burn off the organic particulate to create voids interconnecting the pores to form a permeable fiber reinforced porous ceramic article.